Micro-Machining with Lasers

Producers of microelectronic circuits continue to be challenged to meet the demand for smaller electronic packages. New circuit designs often require thinner ceramic substrates, smaller vias (holes), closer via locations, denser circuit patterns and multi-layer interconnects.
In this environment, conventional green punching or stamping has become quite limited. Punches typically become obsolete between 0.008 and 0.015-in. diameter, and small-diameter punches wear and break too easily. Additionally, intricate profiling and dense patterns are nearly impossible to achieve. And with punching, even the best green ceramic material (0.25% shrinkage) still has too much dimensional shift (± 0.005 in. on a 2 x 2 in. substrate) during firing to be used for these high reliability circuits.
Laser machining solves the shrinkage issue since the material is processed in its fired state. Hole locations drilled by lasers have a typical tolerance of ± 0.002 in., and can be ± 0.001 in. if certain design rules and size limitations are met with the substrate. Some of the more common ceramic substrate materials that are laser machined include 96% alumina, 99% alumina, beryllium oxide and aluminum nitride. These materials offer circuit designers excellent dielectric properties with very good thermal stability and thermal conductivity.

Figure 1. An example of laser drilled vias.

Overcoming Challenges

The laser machining process involves focusing a high-intensity beam of infrared light energy onto the surface of the ceramic substrate. The laser energy vaporizes some of the material in a very small area to create the required patterns or vias. Laser machining permits the profiling of complex, two-dimensional shapes into the ceramic substrate without tool wear, and it allows the formation of vias without the potential for drill wear or breakage.
However, while laser machining provides significant benefits over many conventional machining methods, it is not without potential problems. Some users have experienced material cracking, formation of a brittle recast layer, and obstruction of vias with debris. The key to successful laser drilling in hard, brittle materials like ceramics requires optimization of the laser parameters.
Researchers at Laserage Technology Corp. have addressed these issues by developing high-speed laser drilling techniques to produce vias as small as 0.004 in. (100 microns) at a rate of multiple vias per second. These vias can be multi-beam processed (machined using two or more laser beams simultaneously) to achieve very cost-effective products that meet most thick-film, thin-film and metallization-plated applications. Circuitry in the past 10 years has leaped forward from drilling hundreds to thousands or tens of thousands of vias per substrate, and today’s advanced high-speed laser drilling technology is designed to meet these requirements.
Dense via patterns can also be achieved using laser-drilling techniques. The standard of 1x the material thickness from the edge of one hole to the edge of the next hole continues to be the best design rule. However, sometimes a design mandates that producers “push the envelope.” An edge-of-hole to edge-of-hole spacing of 0.5x the material thickness has been used in many cases without increasing the rate of micro-cracking, which can potentially occur in the ceramic between the vias. Figure 1 shows the dense patterns that can be achieved by laser drilling.
Engineers have also resolved the problem of plugged vias by developing an automated inspection system that verifies 100% obstruction-free vias in the substrate. If an obstruction is found it can be easily removed. This technique has greatly improved the overall cost and reliability of these sensitive circuitries by improving process yields (see Figure 2).

Figure 2. This hearing aid circuitry was produced at a lower cost and with improved yields using laser micro-machining. Photo courtesy of Beltone Electronics, Chicago, Ill.

Improving Adhesion

A natural byproduct of laser machining ceramic is an ultra-thin, fragile “columnar layer” on via walls. For applications that use low-fritted (low glass content) thick-film inks for metallization, this layer can adversely affect ink adhesion. To overcome this problem, researchers have developed a proprietary heat-treating process* that securely fuses or bonds the fragile columnar layer to the via wall and improves the adhesion of the metallization. While the process is especially useful in low-fritted applications, it can also be used for any laser-machined process to relieve the stresses in the material, aid in thin-film deposition and/or improve camber (the substrate curve). It is also very useful for any application that includes vias or slots along scribe or score lines, because it prevents the metallization from being peeled away at the ends of the via or slot when the scribe line is broken.

The Future is Smaller

Future developments will lead to laser drilling of increasingly smaller vias. Researchers have demonstrated that via diameters of 0.001 in. (25 microns) can be drilled with currently available laser technology. However, inspection methods need to be improved to measure and facilitate debris-free vias at diameters smaller than 0.003 in. Further developments by the microelectronic circuit producers—such as less viscous metal inks—will also be necessary to metallize these smaller vias.
As these and other challenges are met, laser micro-machining will continue to help producers of microelectronic circuits meet the increasing need for higher quality in smaller packages.

SIDEBAR: CirQon Technologies Puts Laser Micro-Machining into Practice

CirQon Technologies Corp., located in Gurnee, Ill., produces both plated and bonded copper (PBC) and copper plated thick film ceramic (alumina) circuits for the microelectronics industry. As a subsidiary of Laserage Technology Corp., CirQon naturally takes advantage of the company’s advanced laser processing technology to machine its products.
“We use lasers to create vias between the metallurgy on the top of the ceramic substrate and the metallurgy on the bottom. The lasers also allow us to create scribe lines in the ceramic that we can snap by hand, and they allow us to put notches or grooves along the edge of the substrate for mechanical alignment,” says Dr. Timothy Lenihan, vice president of engineering and marketing.
Because the company uses alumina as its substrate material, the conventional method of punching the green ceramic simply isn’t feasible. “Alumina shrinks 15% in the xyz direction during sintering. That makes it difficult to get the holes and spaces exactly where you want them to be,” Lenihan explains. “By using the laser after the ceramic is hardened, we can get excellent alignment.”
While Lenihan admits that the laser machining technology can be expensive, he believes that the benefits make the technology well worth the extra investment. “With lasers, we can offer relatively small vias—down to 3.5 mils [0.0035 in.]—and these smaller vias allow a higher wiring density and therefore greater functionality for the end user. Other technologies, such as printed wire boards and punched ceramic, can’t even get down to 5 or 6 mils without a lot of difficulty.
“Lasers also offer a faster turnaround time than other machining technologies,” he adds. “With punching, for example, it might take up to a week to machine and sinter a substrate. With lasers, this process only takes a matter of minutes. So even though the laser technology is expensive, it allows us to sell more products because of the higher density and faster turnaround we can offer.”
For more information about CirQon Technologies, contact the company at 1394 St. Paul Ave., Gurnee, IL 60031; (847) 360-1900; fax 847-360-1910; e-mail sales@cirqon.com; or visit www.cirqon.com.

SIDEBAR: Better Hearing Through Laser Technology

Several decades ago, everyone knew if you were wearing a hearing aid. The only available models were large and awkward, and they weren’t always entirely reliable. Today, many hearing aids are practically invisible and feature highly accurate digital technology that automatically adjusts based on the level of sound.

According to Gerald Chudoba, plant manager for Beltone Electronics Corp., Chicago, Ill., these improvements are largely the result of advances in laser micro-machining. The company has been working closely with Laserage Technology Corp. for more than 10 years, helping to refine the laser machining technology. “We keep pushing the envelope of what’s possible with lasers,” Chudoba says. “Today, we can put 64 small circuits on a single 2-3/4 in. x 2-3/4 in. substrate. That’s pretty remarkable considering that when we first started working with ceramic circuit boards, the laser technology could only apply a few circuits to a single substrate.

“Some of the tolerances we have are also quite remarkable—down to 0.0002 in,” Chudoba adds. “The micro-machined vias allow us to actually put the via on the paths and still make a reliable thin-cell connection, which, in turn, allows us to stack the interconnects, and we end up with more of a very small ‘cube’ than what would traditionally be considered a circuit. As a result, our hearing aids have become much smaller while providing higher quality for the end user. The longest dimension of our biggest hybrid model, by far, is probably less than a 1/4 in, and we’ve never had a circuit failure in any of our hearing aids. In fact, our hybrids are probably some of the most reliable hearing aids in the industry.”

Laser machining has also led to productivity advances in manufacturing the hearing aid circuits. “With the laser technology, we’ve been able to implement a batch process that enables us to really cut down on our costs while still achieving the same via shape across the whole array,” explains Chudoba.